Broadband DC-coupled amplifiers are generally designed with differential inputs and outputs for reasons such as power supply (and other common-mode) noise immunity, cancelation of even-order harmonic distortion, cancelation of DC offset terms, increased dynamic range due to swing on both outputs, etc. For interconnect between amplifiers on one die, one package, or even on one circuit board, the expense of the differential interconnect is small compared to the advantages of differential design. However, for interconnect between modules, such as between an active probe and an oscilloscope, the cost of differential interconnects are often prohibitive. Not only would two coaxial cables be required rather than one (adding cost and bulk, and reducing flexibility), but the two would also need to be tightly matched to prevent mode conversion from differential to common-mode and vice versa.
Various passive interconnect structures are known that convert between single-ended and differential signals, often called “baluns” in time-domain applications and/or “180° hybrids” in frequency-domain applications. Broadband, DC-coupled passive baluns are limited to a loss of at least 3 dB, as at DC no energy can be coupled with capacitive or inductive coupling to the “inverted” output, and hence half of the single-ended input power appears as “wasted” common-mode energy at the differential output. (Equivalently, for a balun converting a differential input to a single-ended output, half the differential power in the “inverted” input cannot be coupled to the output, and thus is lost. This symmetry can also be inferred from reciprocity of passive elements with the power loss of a passive balun structure is independent of whether it is used to convert balanced to unbalanced or vice versa.
Generally, baluns are designed for RF applications and little or no consideration is given to the transient response of the balun. The transient response in such device may have substantial pre-shoot or pre-shoot and over shoot. However, in certain application, such as a signal acquisition system having a differential signal acquisition probe coupled to oscilloscope, the transient response of the balun should have little or no pre-shoot. Further, the balun needs to have a wide bandwidth extending down to DC for coupling a wide range of differential signal to the oscilloscope. In addition, the balun should provide compensation for signal losses in the signal cable of the signal acquisition probe system.